The present invention relates generally to surgical procedures for spinal stabilization, and more specifically to instrumentation and techniques for inserting a spinal implant within the intervertebral disc space between adjacent vertebra. More particularly, while aspects of the present invention may have other applications, the invention provides instruments, techniques, and implants especially suited for implanting one or more fusion cages in an unreamed disc space.
There have been an extensive number of attempts to develop an exceptional intradiscal implant that could be used to maintain the height and stability of the disc interspace between adjacent vertebra, at least until complete arthrodesis is achieved. These xe2x80x9cinterbody fusion devicesxe2x80x9d have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are presented in patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In the cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be driven into the intradiscal space with little or no rotation. For example, this type of device is represented in the patent to Brantigan. A combination implant having the ability for threaded insertion or push-in insertion is disclosed in U.S. Pat. No. 5,782,919 to Zdeblick et al.
U.S. Pat. No. 5,484,437 to Michelson discloses a technique and associated instrumentation for inserting a fusion device. As described in more detail in the ""437 patent, the surgical technique involves the use of a distractor having a penetrating portion that urges the vertebral bodies apart and a hollow sleeve having teeth at one end that are driven into the vertebrae adjacent the disc space created by the distractor. These teeth engage the vertebra to maintain the disc space height during subsequent steps of the procedure following removal of the distractor. In accordance with one aspect of the ""437 patent, a drill is passed through the hollow sleeve to remove portions of the disc material and vertebral bone in order to ream the disc space and reduce the endplates to bleeding to produce a prepared bore for insertion of the fusion device. The drill is then removed from the sleeve and a fusion device having a diameter greater than the height of the disc space is positioned within the disc space using an insertion tool.
The device depicted in the Michelson patent is representative of this type of hollow implant which is typically filled with a bone growth inducing substance to promote bone growth into and through the device. This implant includes a plurality of circular apertures which communicate with the hollow interior of the implant, thereby providing a path for tissue growth between the vertebral end plates and the bone growth material within the implant.
One problem that is not adequately addressed by the above prior devices concerns restoring and maintaining the normal anatomy of the fused spinal segment. Naturally, once the disc is removed, the normal lordotic or kyphotic curvature of the spine may be altered. In response to this problem, the adjacent vertebral bodies may be reamed with a cylindrical reamer that fits the particular shape of the implant. In some cases, distraction techniques are used to establish the normal curvature prior to reaming. However, for a cylindrical implant, the over-reaming of the posterior portion is generally not well accepted because of the extensive removal of load bearing bone of the vertebrae. Over time, the implant tends to migrate into the vertebral bodies since the load-bearing surfaces of the endplates are no longer adequate for the implant to support the spinal column loads. This migration is often referred to as subsidence. When an implant subsides into adjacent bone, the disc space can collapse, resulting in potentially adverse consequences to the patient""s health.
Another problem is that when the disc space and adjacent endplates are reamed, the implant must have a height greater than that of the original disc space height to restore the disc space to its normal anatomy. When large implants are bi-laterally inserted in the disc space, the lateral spacing and separation between the implants that can be attained is less than that attainable with smaller implants used in the same disc space. This positions the larger implants closer to the medial portion of the disc space and vertebral endplates, thus increasing the risk of migration into the vertebral bodies and subsidence of the spinal column around the implant. Also, lateral stability of the spinal column is reduced since less support is provided at the hardy bony peripheral ring of the adjacent vertebral bodies. Thus, it is desirable to maintain proper lateral separation of the implants in the disc space so that each implant is bearing on the strongest portion of the vertebral bodies and the lateral stability of the spinal column is maintained.
While the more recent techniques and instrumentation represent an advance over earlier surgical procedures for the preparation of the disc space and insertion of the fusion device, the need for improvement still remains. There remains a need for interbody fusion cages that may be inserted into an unreamed disc space, as well as instruments and techniques for inserting these fusion cages in an unreamed disc space to stabilize the spine. The present invention is directed to these needs and provides convenient methods, instruments, and implants for effective preparation of an unreamed disc space in conjunction with implant placement.
One object of the present invention is to provide instruments permitting placement of a fusion cage in an unreamed disc space. One instrument of the present invention includes a first distractor and a second distractor configured to be inserted in side-by-side relation in the disc space. At least one distractor has a guide surface abutting the other distractor to maintain the distractor spacing as the distractor pair is inserted into the disc space. In a preferred form, the guide surface is offset from the central axis of the instrument.
In one specific embodiment, each distractor has a body portion with a leading end and a trailing end. The body portions distract the disc space and form a channel therethrough as the distractors are inserted. Preferably, at least one of the distractors is provided with a medially extending portion extending from the body portion towards medial area of the disc space. The guide surface is formed by a medial side of the medially extending portion. The medially extending portion can be provided with a height less than that of the body portion. The guide surface of the medially extending portion guides the insertion of a cage into the distraction channel formed through the unreamed disc space remaining after withdrawal of the other distractor.
In another instrument of the present invention, first and second distractors are provided with first and second central spacers. Each central spacer has a width between its guide surface and the body portion of the distractor from which it extends. The central spacers extend medially from the body portion into the disc space so that the guide surfaces are adjacent one another. In one form, the first central spacer has a width that is greater than the width of the second central spacer. The guide surfaces maintain the spacing between the body portions as the first and second distractors are inserted into the disc space. Alternatively, the central spacers are provided with an equal width. In another form, the first and second distractors each further include a lateral spacer having a width that tapers from a maximum width at the trailing end of the body portion to a minimum width at the distal end of the body portion. It is preferred that the body portion have a diameter that is substantially the same as the diameter of the leading end of the fusion cage to be inserted into the disc space. In one preferred form, the guide surface of the central spacer of the first distractor guides the insertion of an implant into the distraction channel formed through the unreamed disc space remaining after withdrawal of the second distractor.
In yet another specific embodiment of the distractors of the present invention, the leading end of the body portion is tapered to facilitate insertion of the distractor into the unreamed disc space. Alternatively, the leading end of the body portion is rounded. It is also contemplated that the top and/or bottom surfaces of the body portion contacting the vertebral endplates may be roughened along a portion of the length of the body portion starting at the trailing end. The roughened surface scrapes the vertebral endplate during insertion and resists migration of the distractors in the disc space. The body portions of the distractors can also include a hollow threaded cylindrical hole or bore to connect the distractor to an obturator or shaft used for inserting and withdrawing the distractors.
In another aspect of the present invention, a method of distracting a disc space is provided. The method comprises providing a first distractor having a first longitudinal axis and a guide surface spaced a first distance from the first axis, and providing a second distractor having a second longitudinal axis. Each distractor is connected with a corresponding shaft, and positioned with the guide surface abutting a side of the second distractor. The first and second distractors are simultaneously inserted to distract the disc space to form a distraction channel.
In one form, the method additionally includes preparing a starter channel at the anterior lip of the disc space adjacent the first distractor. A channel starter instrument is provided with an outer shaft and an inner shaft. A cutting blade is positioned between the outer shaft and the inner shaft. The inner shaft has an end portion received within an opening formed at the trailing end of the first distractor. The cutting blade removes a portion of the endplate thickness at the anterior lip of the vertebral bodies, thus forming a starting channel in the disc space that is coextensive with the channel formed in the disc space by the removed second distractor. The above steps are repeated at the location of the first removed distractor if desired to form a second starter channel.
Yet a further aspect of the present invention provides a method for inserting one or more fusion cages in an unreamed disc space after insertion of the distractors as described above. The second distractor is removed from the disc space, forming a distraction channel in the disc space. A fusion cage, preferably having a root diameter or height approximating the disc space height, is inserted in the disc space. The first distractor guide surface maintains lateral positioning of the cage in the disc space as it is inserted. The first distractor is removed, and a spacer device is secured to the first fusion cage to act as a guide for insertion of a second fusion cage in the distraction channel formed by the first distractor.
In another form, the first and second fusion cages have openings in their top and bottom surfaces adjacent the endplates. A curette or other cutting instrument is placed into the cage, and the bone portion of the vertebral endplates adjacent the openings is removed. The remaining portion of the endplates in contact with the top and bottom surfaces of the cages remains intact to provide a strong bearing surface. Bone growth material is then placed within the fusion cages.
Yet another aspect of the present invention is to provide a fusion cage for insertion in an unreamed disc space. The cage includes a body having a hollow interior extending between a trailing end and a leading end. In one form, the body is threaded and tapered to restore lordosis when inserted in the disc space. The cage has a top surface and a bottom surface positioned in contact with the intact endplates of the vertebrae when the cage is implanted. The body defines a number of openings in the top and bottom surface. Preferably, external threads extend outwardly from the body portion and engage the cage to the bony end plates and harvest disc material and bone from the endplates for deposit through the openings into the hollow interior. In another form, the fusion cage is provided with threads with a swept back profile that increase in depth from the leading end to the trailing end to prevent backout of the inserted cage from the disc space.
One aspect of the invention contemplates providing an interbody fusion cage or device having opposed upper and lower bearing surfaces separated by a height. In one form, the height tapers along the length of the device to match angulation between endplates of adjacent vertebra. In a preferred form, the device includes migration resistance structures intended to limit movement of the fusion device in the disc space. Preferably, these migration resistance structures may include threads, ridges, knurling, spikes, or other surface irregularities extending from the bearing surface. One improvement of interbody fusion devices according to the present invention can be characterized as the spacing distance being substantially equal to the distance between unreamed endplates of adjacent vertebra thereby eliminating the need for removing vertebral endplate bone to form an insertion channel.
In another aspect of the invention, there is provided a method for preparing vertebral endplates through a fusion cage inserted in a disc space. The fusion cage includes at least one opening communicating with the endplate. Bone is removed from the endplate through the at least one opening after insertion of the fusion cage into the disc space.
In one preferred form, the fusion cage is inserted into a disc space having intact endplates. In another preferred form, a cutting instrument is provided that is inserted through the cage and configured to remove endplate bone material through the at least one opening. In one form, the cutting instrument includes a burr for removing bony material. In another form, the cutting instrument includes a curette for removing bony material. In yet another form, the cutting instrument is configured to remove bony material simultaneously from laterally adjacent holes through the fusion cage. In another preferred form, the fusion cage includes a guide at the trailing end of the cage opposite the at least one hole for the maintaining the cutting instrument alignment and facilitating use of the cutting instrument in the fusion cage.
Still a further object of the present invention is to provide a spinal disc space distractor assembly. The distractor assembly has a central axis extending therethrough. Preferably, the assembly includes a pair of distractors positioned in side-by-side relation, the distractors forming a guide surface therebetween. Preferably, the guide surface is offset a distance from the central axis.
The present invention also contemplates a method of preparing a disc space and inserting an implant in an unreamed disc space. The method utilizes one or more of the instruments and implants described above to prepare the disc space for receiving an implant.
Related aspects, features, forms, embodiments, objects and advantages of the present invention will be apparent from the following description.